1. Field of the Invention
The present invention relates to a method for transmitting a word (an information representative of the number of spreading codes) representative of transmission parameters respectively allocated to the mobile stations in communication with a base station of a mobile telecommunication system.
The present invention is concerned with mobile telecommunication systems comprising a number of base stations which can communicate with mobile stations. FIG. 1 shows a base station BTS in communication with three mobile stations MS1, MS2 and MS3. The communication from a mobile station MSi to the base station BTS is done by means of an up-link UL and the communication from the base station BTS to a mobile station MSi is done by means of a down-link DL.
The present invention is also concerned with telecommunication systems wherein different user signals are separated both in time domain and in code domain. An example of such system is the so called UMTS TDD system or W-CDMA TDD system in which the time domain is represented by the TDD-system component and the code domain by CDMA-system component.
2. Discussion of the Background
More particularly, in time-domain, transmission is for example organised based on radio frames constituted of a number N (for example N=15) of timeslots. The same frequency is used for both the up-link (Mobile Station to Base Station) and the down-link (Base Station to Mobile Station). Furthermore, a time-separation is used to differentiate the down-link and the up-link such that a subset of the N available timeslots per frame is exclusively allocated for down-link transmission and the remaining ones for up-link transmission. In a frame, at least one timeslot is always allocated for each down-link and up-link.
In such a system, different user""s signals can be transmitted in separate timeslots, e.g. N different down-link timeslots are allocated to N different down-link user signals. This is the time-domain of the system. Furthermore, several users signals can also be transmitted within one timeslot by using different spreading codes, This is the code-domain mode of the system.
In such a system, all base stations in an area operate synchronously and generally share the same up-link/down-link timeslot configurations.
In both up-link and down-link, user""s data is transmitted in a timeslot arranged in a burst B comprising, as illustrated in FIG. 2, a first data field D1, a general midamble field M and a second data field D2. A midamble is a complex-valued chip sequence and is used by a receiver (the base station BTS in the up-link or a mobile station in the down-link) for channel estimation which is needed for the retrieval of the user""s signals.
In the up-link, each mobile station MSi sends a different midamble m(i), as the base station BTS needs an individual channel estimation for each mobile station transmitting in a particular timeslot.
Note that when a midamble is not explicitly assigned to a mobile station, a default fixed-allocation rule between its assigned spreading code and a particular midamble is used.
In the down-link shown in FIG. 2, generally just one midamble m(j) is used by the base station BTS for all user""s signals within a particular timeslot. The reason is that in the down-link, all users experience just one down-link channel to estimate, e.g. from the base station BTS to itself and ignore those of the other users transmitting in the same timeslot. But in some situation, when more than one channel estimation is needed, more that one midamble can be used by a base station BTS. In this cases, the midamble M results in the summation of all these midambles.
A guard period G can be provided to ensure proper separation in time of consecutive timeslots. Also, signalling bits S can be provided.
In the up-link UL, data of a mobile station MSi is spread to the chip rate by a complex valued spreading code ai (or the spreading codes) which is (are) affected to this mobile station MSi by the system.
In the down-link DL, each data di intended for a mobile station MSi is spread to the chip rate by a corresponding spreading code ai (in 11 to 1k on FIG. 2), the results of all these spreading operations being summed (in 20) to form the data D1 and D2 contained in the burst.
A problem occurs when an advanced detection algorithm such as blind code detection and multiuser detection are used for the retrieval of the user""s signals at the receiver side. With such an algorithm implemented, data bits from all users transmitting in a timeslot are simultaneously decoded and decided at receiver-side. For optimal performance of the algorithm, the receiver needs to know amongst several parameters, the number of spreading codes used in the down-link in a particular timeslot.
Generally, when such an algorithm is implemented at a base station-side, the base station can have a knowledge of these parameters because the radio access network to which it belongs controls their usage.
But, the situation is quite different, when the considered algorithm is implemented at the mobile station in the down-link. A mobile station doesn""t generally know the number of spreading codes which are allocated to the other user""s signals simultaneously present in the same timeslot. This fact seriously impacts the implementation of the algorithm, such the blind code detection and multiuser detection, at the mobile station-side.
It is an object of the present invention to provide a method for a mobile station to determine the number of spreading codes that have been allocated to the other user""s signals simultaneously present in the same timeslot and that are used in the down-link in such a way that this method do not present the underlying problem.
It is a further object of the present invention to provide a method which can be performed without any substantial constraint and, hence, which can be done fast and with only marginal delay.
It is a further object of the present invention to provide such a method that can be carried out in mobile telecommunication system designed in such a way that each mobile station in communication with said base station transmits data in bursts including a midamble or a sum of midambles that are affected to said mobile station and that said or each midamble is used for estimating the channel response between said mobile station and said base station, all said available midambles being derived from an unique basic midamble code by retaining only the elements of said basic midamble code which belong to respective predefined windows shifted one relative to another, said estimations being performed by correlating the received signal with a sequence based on said basic midamble code and channel estimation output being in temporal positions in one-to-one relationship with said available midambles.
The objects of the present invention are achieved by a method for transmitting an information representative of the number of spreading codes that includes the step of:
forming a word, said transmitted word, the content of which is representative of the number of spreading codes allocated,
including in each transmission burst, when data are transmitted from a base station to a mobile station, a general midamble resulting from the sum of selected midambles among all the available midambles, said selection being done by said base station in relation with said transmitted word so that a selected midamble corresponds to a binary element of said transmitted word equal to a first value and a non-selected midamble corresponds to a binary element of said transmitted word equal to second value, considering, at each mobile station side, after having correlated the signal received by said mobile station with a sequence based on the basic midamble code used during the formation of all said midambles, a received word the elements of which are in one-to-one relationship with the temporal positions of the estimations respectively corresponding to said available midambles, an element of said received word being equal to said first value when the corresponding position includes an estimation of the channel between the base station and the mobile station and being equal to said second value when the corresponding position doesn""t, said received word equal to the transmitted word enabling said mobile station to have a knowledge of the number of spreading codes the number of spreading codes allocated to the mobile stations in communication with a base station of a mobile telecommunication system.
In accordance with an additional feature of the present invention, the formation of said transmitted word is done in such a way that to each state formed by a number or all of its bits corresponds a number of the allocated spreading codes.
In accordance with an additional feature of the present invention data of each mobile station in communication with said base station being transmitted by means of a plurality of channels, wherein the formation of said transmitted word is done in such a way that for each channel, to each state formed by a number of its bits identical for each channel, corresponds a number of the allocated spreading codes.
In accordance with an additional feature of the present invention, each of said states is equal to the binary value of the number of the allocated spreading codes.
In accordance with an other feature of the present invention, the relation between a state and a number of spreading codes is done in such a way that the number of significant bits of said state is bigger than needed to express the number of spreading codes that can be allocated. For example, to each position of a bit of said state corresponds the number of spreading codes.
In accordance with an other feature of the present invention, the number of spreading codes are grouped in groups of spreading codes, to each position of a bit of a number of significant bits of said state corresponds the number of spreading codes in a group, the other significant bits being in relation with the groups.
In accordance with an additional feature of the present invention, the numbers are grouped in such a way that to one group corresponds a plurality of numbers, the formation of said transmitted word being done in such a way that to one group corresponds one state of a number or all of its bits.